In Conventional x-ray radiography, x-rays from a nearly point source are directed on the part of the body (organ) to be imaged. The x-rays emerging from the organ are detected to form a two-dimensional image, producing a shadowgram of the part of the body being imaged. The points on the image have a brightness related to the intensity of the x-rays at that point. Image production relies on the fact that different parts of the anatomy absorb different amounts of x-rays. At times, it is necessary to enhance the x-ray absorption of an organ by introducing x-ray absorbers to the body. Examples of x-ray contrast agents are physiologically acceptable organic salts, especially those containing one or more tri-iodo groups in their structure, like 3-acetylamino-2,4,6-triiodobenzoic acid, and 5-[(3-amino-2,4,6-triiodophenyl) methylamino]-5-oxypentanoic acid.
Mammography is a procedure which utilizes x-ray imaging for the examination of the breast. It is used for the detection and diagnosis of breast cancer, for preoperative localization of suspected lesions and for guiding biopsy needles.
A schematic representation of a mammogram 10 is shown in FIG. 1. Generally, mammogram 10 comprises a head 12 and a base 16. Head 12 comprises an x-ray tube 14. Base 16 comprises three parts:                1. A support plate 22 on which the breast rests;        2. A compression plate 20 mounted on a gantry 21 above support plate 22, for compressing the breast firmly against support plate 22; and        3. An image receptor 18, positioned directly below support plate 22. In digital mammography, image receptor 18 is an electronic detector connected to a computer 24.        
X-ray imaging involves placing the breast (not shown) on support plate 22, bringing compression plate 20 down for firm compression of the breast and activating x-ray tube 14. Rays transmitted through the breast strike image receptor 18 where they interact and deposit energy locally, forming an image.
A schematic representation of stereoscopic mammography is shown in FIG. 2. In stereoscopic mammography, head 12, is mounted on a gantry (not shown) that turns on an arc about base 16. As a result, x-ray images can be obtained from different angular views. Images from different angular views can be used to generate a three dimensional location of a suspected lesion.
Mammogram 10 is shown upright, for use with patients that are sitting or standing. Generally, its assembly, including support plate 22 and compression plate 20, can be rotated so as to obtain x-ray measurements when the patient is lying down. Generally, the assembly of mammogram 10, including x-ray tube 12, support plate 22 and compression plate 20 may be rotated at any angle about the horizontal axis, so as to obtain images of the breast from any desired position.
Stereotactic biopsy is a procedure that utilizes stereoscopic mammography for guiding a biopsy needle to the location of a suspected lesion. A core sample tissue is then cut for laboratory examination.
A stereotactic biopsy system comprises a stereoscopic mammogram which has a secondary gantry for positioning a biopsy needle. Typically, the secondary gantry is capable of moving along all axes and in at least one rotational direction. However, a gantry having fewer degrees of freedom may also be used. The secondary gantry is equipped with a spring-loaded biopsy needle which serves as a cutting and collecting device.
Computed tomographic images are cross-sectional images of internal structures of the body, as reconstructed from a large number of measurements of x-ray transmission through a patient, acquired from different views around the patient. The diagnostic CT scanner comprises an x-ray tube with collimation to provide the slice thickness, a linear array of detector elements and a reconstruction computer. Typically, the x-ray tube and the detectors are on a rotating gantry.
In nuclear imaging, a radioactive isotope is injected to, inhaled by or ingested by a patient. The isotope, provided as a radioactive-labeled pharmaceutical (radio-pharmaceutical) is chosen based on bio-kinetic properties that cause preferential uptake by different tissues. The gamma photons emitted by the radio-pharmaceutical are detected by radiation detectors outside the body, giving its spatial and uptake distribution within the body, with little trauma to the patient. Examples of radio-pharmaceuticals used in nuclear medicine are [90Tc]-MIBI and [125I]-albumin.
The basic nuclear-imaging detector is an Anger-type scintillation camera, as disclosed in U.S. Pat. No. 3,011,057, incorporated herein by reference. In practice, more advanced units of this general type are used. Generally, an Anger type camera comprises:                a scintillation crystal (generally, a doped NaI(Tl) crystal).        an array of photodetectors (generally, photo-multiplier tubes, PMTs), to give positional sensitivity; and        coordinate computation circuitry, CCC.        
Each photodetector has an x and a y coordinate. When a photon is absorbed by the scintillation crystal, light is generated at the point of absorption. Several photodetectors receive the light and produce signals. The normalized X′ and Y′ coordinates of the light event are determined by the strength of the signals generated by each photodetector. The total energy of the light event, proportional to the sum of all the signals, is represented by the Z pulse. Only Z pulses within a given range are counted. Other types of gamma cameras are also widely used.
SPECT (Single-Photon-Emission Computed Tomography) is based on conventional nuclear imaging technique coupled with tomographic reconstruction methods, wherein projection (or planar) data of single photons acquired from different views around the tissue are reconstructed, to generate cross-sectional images of the internally distributed radio-pharmaceuticals. SPECT images provide enhanced contrast, when compared with planer images obtained with conventional nuclear imaging methods.
A typical SPECT system consists of a single or multiple radiation detectors arranged in a specific geometric configuration and a mechanism for moving the radiation detectors around the tissue to acquire data from different projection views.
PET (Positron Emission Tomography) uses as radio-pharmaceuticals biological molecules that carry a positron-emitting isotope (e.g., 11C, 13N, 15O, 18F, 52Fe). Within a short period (a few minutes), the isotope accumulates in the area of the body for which the molecule has an affinity. For example, glucose labeled with 11C accumulates in tumors where it is used as a source of energy. The radioactive nuclei decay by positron emission, and the ejected positron combines with an electron almost instantaneously. The two particles undergo annihilation and their combined mass of 1.022 MeV is divided between two 0.511 MeV photons that fly away in opposite directions from one another. PET is based on the coincident (simultaneous) detection, by two opposite detectors, of two 0.511 MeV photons. The source of the photons is along the line connecting them. Data acquired from different views around the tissue are reconstructed, using one of various known image reconstruction methods, to generate cross-sectional images of the internally distributed positron-emitting isotope.
A typical PET system consists of at least one pair of radiation detectors situated opposite to each other and a mechanism for moving the radiation detectors around the tissue to acquire data from various projection views.
Impedance imaging is a procedure which relies on variations in electrical impedance of tissue as suggestive of the possibilities of lesions. U.S. Pat. Nos. 4,291,708 and 4,458,694 and the article, “Breast Cancer Screening by Impedance Measurements,” by G. Piperno et al., Frontiers Med. Biol. Eng., Vol. 2 pp. 111–117, the disclosures of which are incorporated herein by reference, describe systems for determining the impedance between a point of the surface of the tissue and some reference point on the body of the patient. With the use of a multi-element impedance probe, an impedance image of a tissue such as a breast can be generated and used for the detection of tumors, especially malignant tumors.
The multi-element impedance probe may be constructed as a series of flat, conducting, sensing elements, mounted onto a PVC base or some other flexible, nonconductive substrate. A lead wire is connected between each of these elements and detector circuitry. Impedance measurements between the elements and an electrode attached to a remote part of the body are used to determine impedance variations in the tissue, using signal processing circuitry. Alternatively, two multi-element impedance probes may be used, and impedance measurements between them are used to determine impedance variations in the tissue.
In general, impedance imaging involves the following procedure:                1. A reference electrode is held by the patient (or attached to some part of her body);        2. A multi-element impedance probe is placed on the tissue whose impedance is to be imaged;        3. A signal is applied via the reference electrode; and        4. The resulting current (or voltage) response across the tissue is measured by each sensor of the multi-element impedance probe and fed to an electrical impedance scanning device which generates an impedance image.        
U.S. Pat. No. 5,810,742, “Tissue Characterization Based on Impedance Images and on Impedance Measurements,” the disclosure of which is incorporated herein by reference, describes a multi-element impedance probe for the identification of tissue type from impedance images. U.S. Pat. No. 5,810,742 also describes mammography systems utilizing two or more probe arrays on the breast. PCT application PCT/IL00/00127, entitled “uniform, Disposable Interface for Multi-Element Probe,” filed Mar. 1, 2000, the disclosure of which is incorporated herein by reference, describes an interface sheet or other structure to be used in conjunction with a multi-element impedance probe.
U.S. patent application Ser. No. 09/460,699, now U.S. Pat. No. 6,560,480, entitled “Location of Anomolies in Tissue and Guidance of Invasive Tools Based on Impedance Imaging”, the disclosure of which is incorporated herein by reference, describes impedance imaging methods for determining the depth of a lesion within an organ of a patient. The position of an anomaly, including its depth, may be determined from a plurality of impedance maps, obtained by systematically mapping the surface of the organ. Alternatively, two multi-element impedance probes are used, one serving as an exciting electrode, producing electrifying signals, the other serving as a passive sensor, wherein the first impedance probe produces a dipole in the organ. The characteristics of the dipole around the anomaly are indicative of the distance from the source of the dipole to the anomaly. Sometimes, a minimally invasive tool—an impedance-guided biopsy needle, is used, together with an external, sensing probe. As the impedance-guided biopsy needle approaches a lesion, it generates an electrifying signal. Since the lesion's response to the electrifying signal is different from that of the surrounding tissue, the image formed by the sensing, external probe can be used to monitor, manually or automatically, the movements of the impedance-guided biopsy needle toward the lesion. When the needle touches or enters the lesion, the direct electrification of the lesion by the needle induces a detectable change in the signals due to the lesion, which serves to confirm that the needle has indeed reached the lesion, whereupon, a core sample is taken.
Contrast agents are compounds that may be administered to the patient to enhance the contrast between a particular organ of interest and the surrounding tissue. Contrast agents are sometimes used with x-ray imaging, with MRI, and may also be used with impedance imaging. U.S. Pat. No. 5,733,525, whose disclosure is incorporated herein by reference, provides a comprehensive list of contrast agents suitable for impedance imaging, many of them are also useful as x-ray contrast agents. Examples are the aforementioned physiologically acceptable organic salts containing one or more tri-iodo groups in their structure. Other examples, suitable for impedance imaging but not suitable for x-ray imaging, are complexed paramagnetic metal ions such as Fe, Cr and Mn.
To minimize the uncertainty in deciphering the results of any imaging procedure (modality), it is sometimes desirous to compare results of the different modality and seek agreement between them. However, spatial registration may be a problem in the superposition of the images. This is especially true with regard to soft tissue, such as a breast. The breast can change its shape and orientation between imaging sessions. If one conducted a mammography of the breast, using mammogram, and an impedance measurement of the breast moments later, on some other apparatus, it would be very difficult to compare the results exactly; a lesion, if it existed, would be likely to move relative to the imagers, between procedures. Coincident spatial registration is also useful in the guiding of a biopsy needle for core sampling.